Wouldn't make a difference to the prosecutor, and in fact, trying to get witnesses to lie under oath is standard practice. The police are also allowed and even encouraged to lie to you during questioning.
I'm not talking about police tactics, I'm talking about obstruction of justice. I'm not a lawyer, but I watch Law and Order on TV -- don't they have some kind of "tainted fruit" argument that throws out evidence if it was found due to illegal circumstances? If Woodward spoke up sooner, Libby might not have been put in the position to make contradictory statements. Couldn't Woodward's silence have caused a potentially innocent man to be put into a situation that he would not otherwise have found himself in?
-PJ